Glass plate surface heating unit with even temperature distribution

ABSTRACT

A glass-ceramic plate surface heating unit having a heat spreader plate of high thermal conductivity bearing against the underside of the glass to create an even temperature distribution. Means are provided to hold the spreader plate firmly against the glass plate. A metal-sheathed heating element of high watts density is attached to the underside of the heat spreader plate. Reinforcing means are present beneath the spreader plate to prevent warpage. A temperature-limiting means is combined with the heat spreader plate to limit the maximum temperature to which the glass ceramic plate is exposed. The heat spreader plate is grounded to provide a safety precaution since the glass ceramic plate becomes more electrically conductive at higher temperatures.

United States Patent Inventor Bohdal} Hurko 3,191,003 6/1965 Yohe 219/450 bouisvllle, y- 3,355,575 11/1967 Bassett, .Ir. etal 219/464 1 PP N9 58,110 3,445,630 /1969 Ulam 219/438 Flled J y 24, 1970 3,482,079 12/1963 Hurko 219/463 Patented Nov. 23, 1971 3,519,798 7/1970 Walther 219/439 1 1 Asslsnee General Electric p y 3,569,672 3/1971 Hurko 219/464 Primary Examiner- Volodymyr Y. Mayewsky 54] GLASS PLATE SURFACE HEATING N WITH Attorneys-Richard L. Caslin, Francis H. Boos, Jr., Frank L.

EVEN TEMPERATURE DISTRIBUTION Neuhauser and Oscar Weddell 10 Claims, 5 Drawing Figs.

[52] [1.5. CI A glass ceramic plate urface heating unit hay. 2 165/185, 219/449, 21 /4 1, 21 ing a heat spreader plate of high thermal conductivity bearing 21 /530 against the underside of the glass to create an even temperal Int-Cl ture distribution. Means are provided to hold the spreader Field of Search 219/449-450. plate firmly against the glass plate. A metal-sheathed heating 4 430, element of high watts density is attached to the underside of 126/400 the heat spreader plate. Reinforcing means are present beneath the spreader plate to prevent warpage, A tempera- [56] References Cited ture-limiting means is combined with the heat spreader plate UNITED STATES PATENTS to limit the maximum temperature to which the glass ceramic 1,769,752 7/1930 Pais 126/400 plate is exposed. The heat spreader plate is grounded to pro- 2,5 7 10/1 5] Osterheld 219/462 X vide a safety precaution since the glass ceramic plate becomes 3,086,101 4/1963 S cofield 219/464 x more electrically conductive at higher temperatures.

2o 62, I0 I 34 5| 1 1 6 52 1 I as PATENTEnunv 23 I97! WARPED PAN.

No LOAD lzso' FLAT PAN I l 3 6 9 s'z \5 TIME ZOO

FIG.

TEMPERATUR 15 WARF'ED PAN Fucr PAN TIME Mm.

INVENTOR.

H AN HURKO BY M H \s ATTORNEY BACKGROUND OF THE INVENTION The average housewife is becoming more interested in having appliances of more pleasing appearance designs around her home, as well as enjoying the improved ease of cleaning household kitchen appliances. In the matter of cleaning the oven of the range, there has been widespread acceptance of the pyrolytic, self-cleaning oven such as is taught in U.S. Pat. no. 3,121,158 of the present inventor. The solution of the problem of maintaining a clean oven has turned the attention of the housewife to the difficulty of keeping the cook top clean. A standard electric cook top is usually provided with a plurality of metal-sheathed electrical resistance heatingelements lwhich are each wound in the form of a spiral coil and positioned in an opening formed in the cook top. Each heating element is adapted to support a cooking utensil thereon. These metal-sheathed heating elements are cleaned automatically of food soil due to the high temperatures they are allowed to reach once they are energized, but it is possible for spillovers to drainthrough the heating element and accumulate in a collecting pan located beneath the cook top from which they must be cleaned.

In order to reduce this cleaning problem, entire cook tops or in some cases individual solid plate surface units have been proposed in which the exposed surface is formed of glass ceramic plate. In particular, a generally milk white, opaque, glass ceramic or crystalline glass material has been suggested for this use. Examples of such material are the types of crystalline glass sold under such trademarks as PYROCERAM, CER-VIT, and HERCUVIT. The opaque crystalline glass, because of its smooth top surface of almost ground glass appearance, not only presents a pleasing appearance, but it is also readily cleanable and it does not permit the drainage of spillovers therebeneath. There has developed a rather serious problem in obtaining satisfactory heating rates as compared with those available from the traditional, exposed, metalsheathed electrical resistance heating elements, or gas surface burners. One difficulty encountered is the rather poor quality of thermal conductivity through the glass ceramic material. Such a material is used widely as a thermaland Electrical-insulating material rather than as in the present case as a thermal conductor. Heat does not readily diffuse laterally through the glass plate, and during a cooking operation heat will flow to the utensil only near the points of contact. The rest of the heated area will become very hot. Moreover, the glass ceramic plate has a rather large heat capacity, such that when the power is cut off to the heating element it takes a relatively long time to'cool the glass ceramic plate down to room temperature. Also, this type of glass platebecomes increasingly more electrically conductive at the higher temperatures so that this might create a safety hazard in the event an open-coiled heater were employed as the electrical heating means.

The principal object of the present invention is to provide a high-speed, solid plate, surface heating unit with a glass ceramic appearance plate that has an even temperature distribution by use of a heat spreader plate and is still capable of efiicient operation when usedwith cooking utensils having irregular bottoms supported on the surface unit.

A further object of the present invention is to provide a glass surface heating plate of the class described with .an insulated electrical resistance heating element of high watts density in combination with a temperture-limiting means so as to be able to obtain fast heatup rates without overheating the glass surface heating plate by controlling its maximum operating temperature.

A further object of the present invention is to provide a glass surface heating plate that is supported on a heat spreader plate of high thermal conductivity so as to create an even temperature distribution, there being releasable spring means for holding thetwo parts together.

A further object of the present invention is to provide a glass plate surface heating unit with a grounding means to prevent electrical current leakage.

A still further object of the present invention is to provide a glass plate surface heating unit of the class described with an even temperature distribution that permits a metal-sheathed heating element of high watts density to operate at relatively lower sheath temperatures due to the efficient thermal coupling between the heating element and the glass plate.

SUMMARY OF THE INVENTION The present invention, in accordance with one form thereof, relates to a solid plate surface heating unit comprising a glass ceramic plate supported on.a heat spreader plate of high thermal conductivity. A metal-sheathed electrical resistance heating element is attached to the underside of the spreader plate. Means are provided for limitingthe maximum temperature to which the glass ceramic plate is exposed. This temperature-limiting means comprises a temperature sensor attached to the spreader plate and communicating with temperature responder that is connected in a power circuit for the heating element. Then when the heat spreader plate rises to a predeterminedtemperature the responder will open the power circuit of. the heating element. This permits the use of a heating element of high watts density which affords a fast heating rate.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be better understood from the following description taken in conjunction with the accompanying drawingsand its scope will bepointed outin the appended claims.

FIG. 1 is a fragmentary cross-sectional elevational view through about one-half of a solid glass plate surface heating unit embodyingthe present invention showing a temperature responder of the temperature-limiting means suspended from the lower left side of the heating unit.

FIG. 2 is a diagrammatic showing on a smaller scale of a glass plate surface heating unit embodying the present invention and showing the power circuit for the heating element of the surfaceuhit provided with a temperature-limiting means to control the maximum operating temperature of the glass plate.

FIG. 3 is a fragmentary isometric view on a slightly enlarged scale showing the side of the glass surface heating unit of FIG.

.1, and illustrating the manner of supporting the heating unit in conjunction with a recessed flange that circumscribes an opening ina cook top, as well-as a trim ring and holddown means for clamping the various elements of the surface unit together and closing and sealing the edge of the heating unit from the passage of liquid therebeneath.

FIG. 4 is a performance chart or temperature graph of the heating coil for a standard metal-sheathed resistance heating unit held in contact with a solid glass plate plotting temperature versus time and showing a set of temperature curves for boiling a quartofwater; the first beingfor a flat pan, the second being for a no-load condition and the third being for a warped pan.

FIG. 5 is another performance chart or graph of a glass plate surface heating unit of the present invention for comparison withthe performance of a standard metal-sheathed resistance heating element in FIG. 4, the first curve being for a flat pan, the second. being for a warped pan and the third being for a no-load condition.

DESCRIPTION-OF THE PREFERRED EMBODIMENTS Turning now to a consideration of the drawings and in particular to FIG. 1, there isshowna cross-sectional elevational view of little more than one-half of a solid plate surface heating unit ltlhaving a top glass ceramic plate 12. This glass ceramic plate is electrically insulating and thermally transmissive as well as being highly wear and thermal shock resistant and resistant to the physical and chemical attacks of foods and liquids which may come into contact with the plate. While the term glass ceramic material or crystalline glass material is used throughout, it should be understood that this invention encompasses other materials with similar characteristics such as quartz, high-silica glass, high-temperature glasses and different ceramic materials. It would be extremely difficult to maintain an even temperature distribution across this glass plate 12 if it were heated directly by an open coil heater or a metal-sheathed resistance heating element of looped configuration. Heat diffuses very slowly laterally through the glass plate, and hence hot spots would be created on the glass surface nearest the areas of contact between the heater and the glass. This type of glass cannot exceed an operational temperature of about l,300 F. at any point, hence the total heat output of a glass ceramic surface heating unit would be reduced if the glass plate is provided with an uneven temperature distribution. In the absence of a temperature-limiting means, the glass plate would have to be underheated in order to avoid damaging the glass plate.

The present invention contemplates the introduction of a heat spreader plate 14 beneath the glass ceramic plate 12. This heat spreader plate is of high thermal conductivity, preferably of thin composite metal sheet material with a thin center core 16 for distributing the heat rapidly over the entire plate so as to obtain a generally unifonn temperature distribution. Such a core would be selected from metals and alloys such as copper, silver and aluminum. Copper has very low strength at temperatures ranging in the vicinity of 1,300" F., and also it oxidizes very readily. Since a copper core sheet 16 is of small thickness, on the order of 0.040 inches, it would tend to warp or deform easily under normal use conditions due to thermal stresses caused by temporary uneven temperature distribution during the preheat period and also due to the high temperatures to which it is exposed. Hence, the core 16 is sandwiched or sealed between two thin, integral skins 17 and 18, each of the thickness of about 0.016 inches. Such skins would be selected from metals and alloys such as stainless steel, nickel and chromium. In any selection of materials it should be borne in mind that the core and skin materials should have matched coefficients of thermal expansion. In order to avoid exposure of the copper on the peripheral edge of the plate, the two stainless steel skins 17 and 18 will be sealed over the edge of the core with a pinching action to protect against corrosion and oxidation. The stainless steel skins 17 and 18 being on the outer surface of the composite plate 14 provide strength to the plate and resist warpage because it combines a high strength with high heat diffusivity, which no single material plate can provide. This thin composite sheet material 14 is of a central copper core 16 and two outer stainless steel skins l7 and 18, and it may be formed of individual sheets which are area welded as by a process of explosive welding, which causes a bonding of the metal sheets along their mating surfaces. See my copending US. Pat. No 3,569,672, which was issued on Mar. 9, 1971.

A thin ceramic coating of porcelain enamel, silica or the like surrounding the composite heat spreader plate 14 will further improve the performance of the surface unit. Such a coating will prevent a metallic contact between the composite plate 14 and the glass plate 12, thus reducing the possibility of etching or the creation of weak spots in the glass. Ceramic coatings have a high emissivity as compared with a metallic surface. If the composite plate does not make a good contact with the glass plate, the ceramic coated plate will radiate heat towards the glass plate. The ceramic coating on the bottom of the composite plate 14 will cause the plate 14 to cool down faster for the same reason.

A metal-sheathed resistance heating element 20 shown of three-loop configuration is brazed to the underside of the heat spreader plate 14. As is well understood by those skilled in this art, such a metal-sheathed heating element 20 would include a central electrical resistance, nichrome heating wire 21 of helical formation that is inserted into a thin metal tube or sheath 23 of inconel, stainless steel or the like. Then the sheath is filled with a suitable electrical insulating and thermally conductive material such as magnesium oxide (MgO) or the like to separate the heater wire 21 from the metal sheath 23. The top surface of the heating element 20 is flattened so as to obtain a good area contact of the metal sheath 23 with the heat spreader plate 14. Moreover, the heat spreader plate 14 serves in the manner similar to cooling fins and keeps the heating element 20 at much lower operating temperatures thus increasing the lifespan of the heating element. Also, because of the high heat diffusivity of the heat spreader plate 14, a shorter heating element 20 may be used. In other words, standard heating elements of the same wattage would have more turns or loops that is necessary in the practice of the present invention. One of the two terminals 22 of the heating element is shown extending down in a vertical direction beneath the heating element 20. This terminal 22 is provided with a cold tenninal end having a spade connector 24 for receiving a slipon connector (not shown) for making an electrical connection therewith as is conventional in this art.

In order to strengthen the heat spreader plate 14, the edge of the plate is provided with a downtumed flange 26 thereby giving the heat spreader plate a configuration similar to an inverted shallow pan. Another means of reinforcing the heat spreader plate 14 is to provide a series of diagonal or radial struts 28 which are arranged edgewise and fastened to the underside of the heat spreader plate and possibly to the sheath of the heating element 20 as by brazing or similar methods. Such strut members 28 may be of many different configurations as there is no way to design them. The theory is to give the heat spreader plate sufficient depth or beam action so that it does not deflect readily under thermal or mechanical stresses.

A reflector pan 30 of rather deep drawn configuration is positioned beneath the heating unit 10. Looking at FIG. 1 it will be seen that the plate-type surface heating unit 10 is positioned in a circular opening 32 formed in a horizontal cook top 34. The edge of the cook top which defines the opening 32 for the surface unit 10 is formed with a recessed ledge 36. It is well to provide some means for holding the surface unit 10 down in place, and this function is provided by a trim ring 38 which has a transverse T-shaped cross section with a first vertical shank portion 40 which is insertable into the gap between the periphery of the heating unit 10 and the vertical side 42 of the recessed ledge 36. The upper edge of the shank-40 is provided with a folded-over crown 44 which overlies both the edge ofthe cook top 34 and the edge ofthe glass ceramic plate 12.

Releasable means must be provided for holding the trim ring 38 in place. For this purpose a series of widely spaced clip members 46 are attached to the shank portion 40 of the trim ring 38 at widely spaced positions around the trim ring. Each clip member 46 is of thin spring material of narrow width, and at its upper end it is provided with a pair of offset fingers 48 which are adapted to extend through mating slots 50 formed in the shank portion of the trim ring 38. The only way to insert the fingers 48 through the slots 50 is to remove the heating unit 10 from the cook top and insert the fingers at a generally perpendicular angle with respect to the shank portion 40 of the trim ring, and then pivot the clip member down against the shank portion 40 as is seen in both FIGS. 1 and 3. These fingets become captured in place due to the small clearance between the shank portion 40 and the vertical side 42 of the cook top edge. An asbestos gasket 51 is captured beneath the crown 44 as a moisture barrier. Each clip member 46 is generally of Z-shape in side view having a generally vertical upper flange 52, a generally horizontal midportion 54 and a wide V-shaped lower portion 56. This V-portion 56 has an apex 58 that is directed generally toward the edge of the ledge 36 of the cook top 34 to serve as a detent member such that when the surface unit 10 is lowered onto the recessed ledge 36 of the cook top 34 the detent 58 tends to engage the innermost edge of the ledge 36 until additional force causes the detent to spring away from the ledge and then snap back beneath the portion 54 presses the reflector ledge to serve as a tight holddown means. The horizontal midpan 30 and the heat spreader plate 14 against the underside of the glass ceramic plate 12.

It is important to limit the operating temperature of the glass ceramic plate 12 to a temperature below about 1,300 F. This can best be done by introducing a temperature-limiting limiting means to the solid plate surface unit of the present invention such that the power to the heating element 20 is cut off when 'the heat spreader plate temperature reaches a predetermined limit. This temperature-limiting means comprises a temperature sensor or elongated bulb 62 which is brazed or otherwise attached to the underside of the heat spreader plate 14 adjacent a turn or loop of the heating element 20, as is best seen in FIG. 1. This sensor 62 would be filled with a high-temperature thermostatic fluid such as sodium potassium (NaK) or the like which would communicate with a temperature responder 64 by means of a capillary tube 66 that is shown diagrammatically as a long dash line. This temperature responder 64 is a single-point, temperature-limiting switch or thermostat set at a critical temperature of about 1 ,250 F This temperature responder is electricallyconnected in a power circuit for the heating element 20 as is shown diagrammatically in FIG. 2. The heating element 20 is shown simply as an electrical resistance loadconnected in a power circuit across lines L1 and L2 by leads 68 and 70. Connected across the leads is a manualselector switch 72 for controlling the power to the heating element. An alternative would be to substitute for this switch 72 an infinite heat switch that would govern the level of power to the heating element. Notice the grounding circuit 73 in FIG. 2 connected to the heat spreader 14. This is important because the glass ceramic plate 12 becomes conductive toward the upper end of the temperature range and this creates a safety hazard to the user. The grounding of the heat spreader plate I4 eliminates this safety hazard.

During the development of this solid plate surface heating unit using the heat spreader plate 14 it was discovered that the surface unit had a unique property due to the high heat diffusivity of the heat spreader plate. The surface unit is able to operate satisfactorily at considerably lower temperatures, and reference is made here to the relative performance charts of FIGS. 4 and 5. FIG. .4 shows temperatures of a standard 900 watt spiral-coiled, metal-sheathed resistance heating unit in contactwith a glass plate, while FIG. 5.relates to the heat spreader plate and glass surface heating unit of l,lOO-watts rating of the present invention. Because in both cases the heaters are in contact with glass ceramic plates, the glass at these contact points will. approach these temperatures after a prolonged operationtime. Looking at FIG. 4, the sheath temperature under no-load" conditions might run to above 1,550 F. for the spiral unit, while in the glass ceramic plate unit of FIG. 5 the maximum no load temperature condition would be about l,300 F. In the event a quart of water is to be boiled in a pan with a warped bottom on the heating unit of FIG. 4, the sheath temperature of the spiral heating element would be about l,700 F while in the glass ceramic plate unit of FIG. 5 and of the present invention the temperature of the heat spreader platerwould be about I, I 80 F. Finally, if a quart of water were to be boiled in a pan with a flat bottom, the max imum temperature of the spiral unit would be about l,290 F. while in the plate unit of the present invention the temperature of the spreader plate would be about 850 F. Also, notice that the times for boiling the water are shorter in FIG. 5 than they are in FIG. 4, because a higher wattage is used and the system of FIG. 5 is more efficient. In other words, there is a much better thermal coupling between the cooking utensil and the heat source of the present invention than there would be between a glass ceramic unit if a spiral-coiled heating element were used without the heat spreader plate. This phenomenon permits the design of a low-temperature, highspeed surface unit. For example in a 6-inch diameter glass plate unit without a temperature-limiting means the wattage of the heating element would be limited to approximately 800 watts in order not to overheat the glass plate. On the contrary,

with the incorporation of the temperature-limiting means a 1,600 watt heater may be used successfully. Such high wattage would provide much shorter time to preheat the plate 12. The curves shown in FIG. 5 would have a much steeper slope, than for a l, l OO-Watt unit as tested.

Having described above an important contribution in the art of solid plate, surface-heatng units, it will readily be apparent to those skilled in this an that the present glass ceramic plate would be capable of operating with an even temperature distribution and with more total heat output, and with a fast heating rate. The glass plate would provide a better performance with a warped pan than glass plate units of the prior art. The glass plate units of the prior art would be subject to hot spots when used with a warped pan. The heat tends to flow from the glass plate to the pan through the points of contact and colder spots are created at these points. However, the warped bottom acts as a reflector of heat turning the heat back toward the glass plate and creating hotter areas. The heat spreader plate functions as a temperature equalizer causing heat to flow from the hotter areas to the colder spots. The use of the temperature-limiting means permits the use of higher wattage heaters which reduces the preheat time considerably, as well as the time to boil a quart of water which is considered as a standard heating load for testing purposes. The glass plate is supported by the heat spreader plate and does not require an insulation pad for support, therefore, the heat is free to flow downward. When the power is cutoff, the heat spreader plate 14 and the reinforcing struts 28 act as cooling fins and heat radiates downward from the glass plate. This cooling-down action would be even more effective, if a forced cooling system were used.

The improved cooling-down performance causes a slight drop of the surface unit efficiency because there will be higher heat losses. However, this drop in efficiency is not very significant, because of the nature of glass plate heaters. Glass operates at maximum temperatures that are much lower than the sheath temperature of-metal-sheathed heating elements, and the heat losses are an exponential function of temperature. Radiant heat losses are directly proportional to T". With a temperature-limiting means, slightly higher heat losses do not have .anyeffect on the speed of the unit or time to boil because the heat transfer to the utensil is a function of the glass plate temperature, and this temperature will be maintained in the present invention by supplying more power. It is true however that the power will be applied longer during the cycling period.

This glassplate unit is adaptable to an automatic surface unit because the even temperature distribution allows the plate to be sensed effectively at any point. As an alternative, the glassplate may be fitted with a central hole for insertion of a spring-biased sensing element that is in direct contact with the bottom of a utensil.

The heat spreader plate provides a solid support for the glass plate so a thinner glass plate with a lower thermal mass is used. This surface unit is adaptable tothin cook top units as the surface unit requires no more than 1% inches of cook top depth. A- modification of this invention would be the substitution of a gas burner means beneath the heat spreader plate 14 in place of metal-sheathed electrical resistance heating element 20. However, it is felt that this invention may have more importance when incorporated with an electrical heater because the-heat is so concentrated in an electrical heater. The flame from a gas burner reduced depending on the need.

Modifications of this invention will occur to those skilled in this art, therefore, it is to be understood that this invention is not limited to the particular embodiments disclosed, but that it is intended to cover all modifications which are within the true spirit and scope of this invention as claimed.

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

l. A surface-heating unit comprising a plate of glass ceramic material supported on a plate of composite metal sheet means may be enlarged or material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidationand corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, an insulated electrical resistance heating element attached to the underside of the composite plate, a reinforcing member of open framework attached to the underside of the composite plate to prevent warping thereof, a reflector pan supporting the assembly, a releasable trim ring surrounding the assembly and adapted to mount the heating unit in an apertured support panel 1, and a temperature-limiting means sensing the temperature of the composite plate and in circuit connection with the heating element to deenergize the heating element at composite plate temperatures somewhere above about l,000 F.

2. A surface-heating unit as recited in claim 1 wherein the temperature-limiting means includes a temperature sensor in close thermal contact with the underside of the composite plate and a temperature responder that is supported from the said reflector pan, and connecting means joining the temperature sensor to the temperature responder.

3. A solid plate, surface-heating unit comprising a top plate of glass ceramic material supported on a plate of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidationand corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, an insulated electrical resistance heating element of looped configuration attached to the underside of the composite plate, reinforcing means braced beneath the composite plate to prevent warping thereof, and fastening means for holding the top glass ceramic plate and the composite metal plate firmly together, and a temperature-limiting switch means carried by the unit and having a temperature-sensing probe held in thermal relation ;with the composite plate, said temperature-limiting switch means being adapted to be connected in a power circuit for the heating element to ensure that the glass ceramic plate does not become overheated, whereby the heating element may be of higher wattage than a heating unit without the temperaturelimiting switch so as to reduce the preheat time it takes for the glass ceramic plate to reach a predetermined temperature. 4. A solid plate, surface-heating unit as recited in claim 3 wherein the composite metal plate distributes the heat from the heating element evenly across the top glass ceramic plate, while in the event of the presence ofa warped cooking utensil on the unit, the heat will flow from the hot spots on the top plate to the cold spots by means of the composite metal plate.

5. A plate-type, surface-heating unit comprising a thin plate of glass ceramic material, a thin flat heat spreader plate bearing against the underside of the glass ceramic plate, said heat spreader plate being of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidationand corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, a metal-sheathed resistance heating element of looped configuration attached to the underside of the heat spreader plate, reinforcing means attached to the underside of the heat spreader plate to prevent warpage at elevated temperatures, and temperature-limiting means for controlling the maximum temperature of the glass ceramic plate, said temperature-limiting means comprising a temperature sensor associated with the heating element, and a single-point temperature responder connected to the sensor, and circuit means adapted to join the responder to the heating element such that when the heat spreader plate rises to a predetermined temperature the responder will open the power circuit to the heating element.

6. A glass ceramic surface unit as recited in claim 5 with electrical grounding means joined to the heat spreader plate.

7. A glass ceramic surface unit as recited in claim 5 with a high emissivity coating such as porcelain enamel on the top surface of the said heat spreader plate.

8. A plate-type, surface-heating unit as recited in claim 5 wherein the heating element is of a higher watts density for the diameter of the heating unit than prior heating units that are not protected with temperature-limiting means so as to obtain high heating rates.

9. A plate-type, surface-heating unit as recited in claim 8 wherein the said heat spreader plate has a high coefficient of thermal conductivity and acts as a temperature equalizerfor the glass ceramic plate when the heat in the spreader plate flows from the areas of high temperatures to the areas of lower temperatures thereby obtaining substantially uniform temperature distribution across the heated area of the glass ceramic plate.

10. A glass ceramic cook top having at least one heated area, each heated area having a heat spreaci er plate positioned therebeneath and in contact therewith, said heat spreader plate being of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidationand corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, a metal-sheathed resistance heating element attached to the underside of the heat spreader plate, reinforcing means braced beneath the heat spreader plate to prevent warping thereof, and temperature-limiting means for restricting the maximum temperature of the glass ceramic plate, said temperature-limiting means comprising a temperature sensor, the heat spreader plate and a temperature responder associated with the temperature sensor, said temperature responder being adapted to be connected in a power circuit for the said heating element and capable of deenergizing the heating element when the temperature of the spreader plate is above a predetermined temperature, said heating element being a high wattage unit for the size of the spreader plate to obtain high heating rates, said heat spreader plate also serving as a heat diffuser beneath the heating element such that when the heating element is deenergized the spreader plate serves to reduce the cooldown time. 

1. A surface-heating unit comprising a plate of glass ceramic material supported on a plate of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidationand corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, an insulated electrical resistance heating element attached to the underside of the composite plate, a reinforcing member of open framework attached to the underside of the composite plate to prevent warping thereof, a reflector pan supporting the assembly, a releasable trim ring surrounding the assembly and adapted to mount the heating unit in an apertured support panel, and a temPerature-limiting means sensing the temperature of the composite plate and in circuit connection with the heating element to deenergize the heating element at composite plate temperatures somewhere above about 1,000* F.
 2. A surface-heating unit as recited in claim 1 wherein the temperature-limiting means includes a temperature sensor in close thermal contact with the underside of the composite plate and a temperature responder that is supported from the said reflector pan, and connecting means joining the temperature sensor to the temperature responder.
 3. A solid plate, surface-heating unit comprising a top plate of glass ceramic material supported on a plate of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidation-and corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, an insulated electrical resistance heating element of looped configuration attached to the underside of the composite plate, reinforcing means braced beneath the composite plate to prevent warping thereof, and fastening means for holding the top glass ceramic plate and the composite metal plate firmly together, and a temperature-limiting switch means carried by the unit and having a temperature-sensing probe held in thermal relation with the composite plate, said temperature-limiting switch means being adapted to be connected in a power circuit for the heating element to ensure that the glass ceramic plate does not become overheated, whereby the heating element may be of higher wattage than a heating unit without the temperature-limiting switch so as to reduce the preheat time it takes for the glass ceramic plate to reach a predetermined temperature.
 4. A solid plate, surface-heating unit as recited in claim 3 wherein the composite metal plate distributes the heat from the heating element evenly across the top glass ceramic plate, while in the event of the presence of a warped cooking utensil on the unit, the heat will flow from the hot spots on the top plate to the cold spots by means of the composite metal plate.
 5. A plate-type, surface-heating unit comprising a thin plate of glass ceramic material, a thin flat heat spreader plate bearing against the underside of the glass ceramic plate, said heat spreader plate being of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidation- and corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, a metal-sheathed resistance heating element of looped configuration attached to the underside of the heat spreader plate, reinforcing means attached to the underside of the heat spreader plate to prevent warpage at elevated temperatures, and temperature-limiting means for controlling the maximum temperature of the glass ceramic plate, said temperature-limiting means comprising a temperature sensor associated with the heating element, and a single-point temperature responder connected to the sensor, and circuit means adapted to join the responder to the heating element such that when the heat spreader plate rises to a predetermined temperature the responder will open the power circuit to the heating element.
 6. A glass ceramic surface unit as recited in claim 5 with electrical grounding means joined to the heat spreader plate.
 7. A glass ceramic surface unit as recited in claim 5 with a high emissivity coating such as porcelain enamel on the top surface of the said heat spreader plate.
 8. A plate-type, surface-heating unit as recited in claim 5 wherein the heating element is of a higher watts density for the diameter of the heating unit than prior heating units that are not protected with temperature-limiting means so as to obtain high heating rates.
 9. A plate-type, surface-heatiNg unit as recited in claim 8 wherein the said heat spreader plate has a high coefficient of thermal conductivity and acts as a temperature equalizer for the glass ceramic plate when the heat in the spreader plate flows from the areas of high temperatures to the areas of lower temperatures thereby obtaining substantially uniform temperature distribution across the heated area of the glass ceramic plate.
 10. A glass ceramic cook top having at least one heated area, each heated area having a heat spreader plate positioned therebeneath and in contact therewith, said heat spreader plate being of composite metal sheet material with a center core selected from a group of high thermal conductivity metals and alloys such as copper, silver and aluminum, and an outer skin selected from the group of oxidation- and corrosion-resisting metals and alloys such as stainless steel, nickel and chromium, a metal-sheathed resistance heating element attached to the underside of the heat spreader plate, reinforcing means braced beneath the heat spreader plate to prevent warping thereof, and temperature-limiting means for restricting the maximum temperature of the glass ceramic plate, said temperature-limiting means comprising a temperature sensor, the heat spreader plate and a temperature responder associated with the temperature sensor, said temperature responder being adapted to be connected in a power circuit for the said heating element and capable of deenergizing the heating element when the temperature of the spreader plate is above a predetermined temperature, said heating element being a high wattage unit for the size of the spreader plate to obtain high heating rates, said heat spreader plate also serving as a heat diffuser beneath the heating element such that when the heating element is deenergized the spreader plate serves to reduce the cooldown time. 